Speculative Memory Read Reordering in Multi-Level Architecture
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Solution Overview
Problem
Multi-level memory architectures face latency issues due to delays in handling memory requests, particularly cache misses, which can lead to reduced system performance and inefficient use of resources, as lower memory levels process unnecessary requests before cache lookup completion.
Innovation Solution
Speculatively issued memory read requests are reordered ahead of pending requests in a multi-level memory architecture, bypassing request queues if no memory write request is pending for the same cache line, thereby accelerating communication and reducing latency when a cache miss occurs in the higher level memory.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If memory requests are speculatively issued to lower memory level before cache lookup completion, then latency for cache misses is reduced, but lower memory level processes unnecessary requests increasing workload and decreasing bandwidth
Solution Approach 1:
The patent applies preliminary action by speculatively issuing memory requests to the lower memory level before the cache lookup operation completes. This allows the lower memory level to start processing requests in advance, reducing latency for cache misses. The system performs this action preliminarily without waiting for cache lookup completion, accepting that some requests may turn out to be unnecessary.
Solution Approach 2:
The patent applies local quality by selectively reordering only certain memory requests based on their characteristics. Specifically, read requests are prioritized over write requests in the reordering logic, and requests are only speculatively issued when they meet certain criteria (e.g., not blocked by earlier requests). This selective approach optimizes performance for critical requests while managing the overall workload on the lower memory level.
2Device complexity
If memory requests are handled in FIFO order in pipelined architectures, then request processing is simple, but performance-critical requests are stalled waiting for less critical requests
Solution Approach 1:
The patent applies dynamics by making the request processing order adaptive rather than static. The reordering logic dynamically adjusts the priority of requests based on their type (read vs. write) and timing characteristics. Performance-critical read requests can be prioritized and issued to the lower memory level before less critical write requests, even if they arrived later. This dynamic reordering improves throughput without requiring complete redesign of the pipelined architecture.
3Loss of energy
If multiple levels of memory are used to balance speed and capacity, then cost is reduced, but latency increases due to cache lookup delays
Solution Approach 1:
The patent applies preliminary action by initiating memory requests to lower memory levels before cache lookup operations complete. This speculative issuance reduces the effective latency for cache misses, as the lower memory level is already processing the request when it is needed. The multi-level memory structure is maintained for cost efficiency, but the timing of request issuance is optimized to reduce latency.
Data Source
AI summary
A circuit arrangement and method selectively reorder speculatively issued memory read requests being communicated to a lower memory level in a multi-level memory architecture. In particular, a memory read request that has been speculatively issued to a lower memory level prior to completion of a cache lookup operation initiated in a cache memory in a higher memory level may be reordered ahead of at least one previously received and pending request awaiting communication to the lower memory level. By doing so, the latency associated with the memory read request is reduced when the request results in a cache miss in the higher level memory, and as a result, system performance is improved.